Methods and Systems for Chemical Composition Measurement and Monitoring Using a Rotating Filter Spectrometer

ABSTRACT

The invention relates to methods and systems for measuring and/or monitoring the chemical composition of a sample (e.g., a process stream), and/or detecting specific substances or compounds in a sample, using light spectroscopy such as absorption, emission and fluorescence spectroscopy. In certain embodiments, the invention relates to spectrometers with rotating narrow-band interference optical filter(s) to measure light intensity as a function of wavelength. More specifically, in certain embodiments, the invention relates to a spectrometer system with a rotatable filter assembly with a position detector rigidly attached thereto, and, in certain embodiments, the further use of various oversampling methods and techniques described herein, made particularly useful in conjunction with the rotatable filter assembly. In preferred embodiments, the rotatable filter is tilted with respect to the rotation axis, thereby providing surprisingly improved measurement stability and significantly improved control of the wavelength coverage of the filter spectrometer.

RELATED APPLICATION

This Application claims benefit of U.S. Provisional Patent ApplicationNo. 61/084,985 filed on Jul. 30, 2008, the text of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to spectroscopic methods and systems.More particularly, in certain embodiments, the invention relates tomethods and systems for measuring and/or monitoring the chemicalcomposition of a sample (e.g., a process stream), and/or detectingsubstances or compounds in a sample, using light spectroscopy.

BACKGROUND OF THE INVENTION

Several chemical composition measurement devices using lightspectrometers are currently commercially available. Examples of thetypes of spectrometers currently used include Fourier transform infraredspectrometer (FTIR), dispersive spectrometer (spectrograph ormonochromator) and linear variable filter (LVF) spectrometer. FTIR baseddevices use Michelson interferometers and have generally been consideredto provide the highest performance, due to their high opticalthroughput, which enables high-sensitivity measurements. In contrast,dispersive and linear variable filter spectrometers have significantlylower optical throughput and thus lower sensitivity performance.However, dispersive and linear variable filter spectrometers generallyprovide simpler and more rugged instrumentation, and are less expensiveto manufacture.

Another type of chemical composition measurement and monitoring devicethat is widely used, in particular, in the field of gas monitoring, isnon-dispersive infrared (NDIR) devices. These devices use fixednarrowband optical filters to select a particular wavelength bandregion. They have high optical throughput, rivaling that of FTIR baseddevices, and thus provide high-sensitivity measurement. This type ofdevice, however, is generally not considered to be a spectrometer, as itdoes not measure light intensity as a function of wavelength; rather, itprovides a single measurement value corresponding to the light intensityat a particular wavelength band. For this reason, each device (employingone filter, one photo-detector and one light source) can only measureone compound. Therefore, such devices are not considered to be chemical“composition” measuring devices.

The transmitted wavelength band of a narrowband optical filter, such asthat used in NDIR instruments, can be varied or “tuned” by varying theangle of incidence (U.S. Pat. No. 4,040,747 to Webster, 1977 and U.S.Pat. No. 2,834,246 to Foskett, 1958, both of which are incorporatedherein by reference). Such methods enable the measurement of opticalsignals from multiple wavelengths or wavelength bands using only asingle optical filter, light source and detector, thus potentiallycreating a simple, low-cost, high-throughput spectrometer.

One method of varying the incident angle is to continuously rotate thefilter in one direction and capture the data at the relevant angularpositions. This type of continuously-rotating filter spectrometers hasbeen described (U.S. Pat. No. 4,040,747 to Webster, 1977, U.S. Pat. No.2,834,246 to Foskett, 1958, U.S. Pat No. 5,268,745 to Goody, 1993, U.S.Pat No. 7,099,003 to Saptari, 2006, all of which are incorporated hereinby reference). However, these devices have not been in significantcommercial use. FTIR spectrometers, grating based spectrometers and LVFspectrometers are still by far the most commonly used hardware forchemical composition monitoring, despite the potential advantages forthe rotating tunable filter instruments.

There are weaknesses of the previous rotating tunable filter systemswhich prevent them from being used in a commercial setting as chemicalcomposition measuring or monitoring devices. For example, these systemslack measurement stability and robustness due to wavelength instability,spectral interferences, environmental variations and/or instrumentalchanges. Such systems also lack versatility, in particular, in that theyare not able to provide wide spectral coverage. Furthermore, there aredifficulties in volume manufacturing, in particular, difficulties inproducing reproducible instruments that are interchangeable without eachinstrument requiring empirical sample based calibration.

SUMMARY OF THE INVENTION

The invention provides methods and systems for measuring and/ormonitoring the chemical composition of a sample (e.g., a process streamin an industrial setting), using a spectrometer with rotatingnarrow-band interference optical filter(s). In preferred embodiments,the spectrometer system features a rotatable filter assembly with aposition detector rigidly attached thereto, providing more accurate androbust detection. The rotatable filter is preferably tilted with respectto the rotation axis, thereby providing surprisingly improvedmeasurement stability and significantly improved control of thewavelength coverage of the filter spectrometer. Also, in certainembodiments, the invention includes methods of using such spectrometersfor measuring and monitoring chemical composition of compounds in gas,liquid and/or solid forms, for example, in both laboratory and non-lab(e.g., industrial) settings.

Various oversampling methods and techniques are also presented herein,which are found to be particularly useful when employed in conjunctionwith a spectrometer with the rotatable filter assembly feature asdescribed herein. In certain embodiments, the invention includes methodsof using such spectrometers for measuring and monitoring chemicalcomposition of compounds in gas, liquid and/or solid forms, for example,in both laboratory and non-lab (e.g., industrial) settings.

In certain embodiments, the invention provides a rotating filterspectrometer for chemical composition measurement and monitoring,employs one or multiple light sources, one or multiple photo-detectors,one or multiple narrow-band optical interference filters, a DC motor, aposition encoder, an analog-to-digital conversion device, and acomputing unit. In preferred embodiments, the narrow-band opticalfilter(s) are rigidly mounted on a rotating mechanical assembly drivenby a DC motor. The rotating filter assembly is positioned relative to acollimated light beam from the light source such that the axis ofrotation is perpendicular to the light beam or, preferably, positionedsuch that the axis of rotation is slightly non-perpendicular to thelight beam, such non-perpendicular conformation resulting insurprisingly improved measurement stability due to apparent suppressionof back-reflected or stray light, and resulting in significantlyimproved control of the wavelength coverage of the filter spectrometer,given the filter characteristics and the angular coverage of themechanical system.

In preferred embodiments, the rotating filter assembly rotatescontinuously in one angular direction. A rotary positional encoder isrigidly attached to the rotating filter assembly such that there is norelative displacement or mechanical “compliance” or “play” between itand the rotating filter assembly. The digital pulses generated by theencoder during motion are used to clock the analog-to-digital conversionof the signal collected by the photo-detector. Furthermore, the encoderand its processing electronics are designed, configured and/or selectedin such a way that it produces significantly more pulses-per-rotationthan what is required to accurately measure the relevant spectralfeatures. The spectral signal is over-sampled. A convolution algorithmis then preferably applied to digitally process the recorded spectrum toenhance wavelength stability or repeatability and to improve spectralsignal-to-noise ratio.

In certain embodiments, inherent or deliberately-introduced spectralfeatures are used to lock the relative position of the encoder withrespect to the rotating filter assembly. The spectral features may bethose due to the spectral characteristics of the light source, system'soptical components, and/or the sample compound itself. Such methodsensure wavelength stability despite alignment changes due to mechanicalforces or temperature changes.

A variable gain amplifier is preferably employed to automatically adjustthe photo-detector signal amplification gain in real-time. The gainprofile may be scheduled based upon the location of the rotating filterassembly, or updated automatically based upon the magnitude of thereceived signal. Such a feature enables measurement of distinctlydifferent spectral regions, such as measurement at the near infrared andthe mid infrared regions simultaneously, without saturating theanalog-to-digital circuitry. Similarly, the light source intensity maybe varied to further optimize the measurement dynamic range and tobetter observe weak spectral features.

In certain embodiments, multiple regression regions and calibrationmatrices, combined with cross-analysis, are used to enhance robustnessand accuracy of multi-compound measurement as well as measurement inhighly complex sample matrices. Each calibration matrix can be optimizedfor a particular target compound or features of the target compounds.The effects of nonlinearities can be significantly suppressed.

In certain embodiments, an adaptive regression analysis is employed toaccount for spectral baseline variations that may have complex shapesdue to the filter's non-linear wavelength-angle function. The algorithmautomatically and continually updates to compensate for the baselinevariations, as well as other spectral variations such as those due tolight interactions with unknown, interfering compounds.

A differential measurement may be employed in applications monitoringcertain processes or reactions, for example, where the input and outputstreams are available for analysis. The method suppresses the effects ofinstrumental and environmental changes, as well as minimizes the effectsof sample background interferences.

Embodiments of the invention provide methods, systems (includingapparatus) for chemical composition measurement and monitoring in gas,liquid and/or solid samples which utilize a single or multiplecontinuously rotating narrow-band filters. In certain embodiments, theinvention provides negligible wavelength instability or drift due tovarious environmental disturbances such as vibrations and temperaturevariations over a long period of time. Embodiments of the invention alsoprovide wide spectral or wavelength coverage with optimum use of themeasurement dynamic range throughout the analysis range, suitable forsimultaneous measurement and/or monitoring of multiple compounds. Thesystems effectively compensate for spectral baseline instability and canbe built and manufactured consistently (without requiring extensive,individual-machine calibration) and relatively inexpensively.

The systems and methods can be used for continuous monitoring of gas,liquid, and/or solid chemical composition (% levels), for example, formonitoring production throughput and quality, e.g., in process streams.They can also be used for gas, liquid, or solid phase trace speciesmonitoring (ppm or ppb levels), for example, impurity detection andmonitoring, e.g., in process streams. Embodiments may also provideambient monitoring for safety purposes. The systems and methodsdescribed herein may be applied, for example, in the petrochemical,bioreactor (biofuel), pharmaceutical, food and beverage, specialtychemical, and/or alternative energy industries.

For example, an embodiment of the invention provides combustion processmonitoring (e.g., alternative energy production using a bioreactor) forthe monitoring of any one or more of the following process gases: CO,CO₂, O₂, CH₄ (methane), N₂O (nitrous oxide). In other embodiments, theinvention provides systems and/or methods for monitoring trace levels(e.g., ppm or sub-ppm) of sulfur compounds (e.g., dimethyl sulfide,dimethyl disulfide, carbonyl sulfide, hydrogen sulfide, etc.) in anatural gas line, for example, in a fuel cell-based power plant. In yetanother embodiment, the invention provides a system and/or method formonitoring trace levels (e.g., ppm or sub-ppm) of CO, CO₂, H₂O(moisture), THC (total hydrocarbon) gases in N₂ or He, for example, forspecialty chemical manufacturers. Other example applications of themethods and systems of the invention include the monitoring of tracewater in fuels, the monitoring of aqueous alcohols, and the monitoringof glucose, lactate, ammonia, and/or glutamine during fermentationprocesses.

In one aspect, the invention provides a spectroscopic system fordetecting electromagnetic radiation that has passed through or isreflected from a sample, the system including an electromagneticradiation source and a rotatable filter assembly configured to filter abeam of electromagnetic radiation produced by the electromagneticradiation source, where the assembly includes one or more bandpassoptical interference filters, and where the rotatable filter assembly isconfigured to rotate to provide continuous adjustment of the incidentangle of the electromagnetic beam onto the one or more opticalinterference filters, thereby providing a continuous wavelength sweep ina single wavelength band or multiple wavelength bands. One or more ofthe bandpass filters is configured such that the surface of the filteris not exactly perpendicular to the electromagnetic beam at any pointduring the continuous adjustment (e.g., the surface is displaced fromperpendicular by up to about 3 degrees, by up to about 5 degrees, by upto about 10 degrees, by up to about 20 degrees, or by up to about 30degrees). The system also includes a motor coupled to the rotatablefilter assembly and an electromagnetic radiation detector configured todetect electromagnetic radiation that has passed through or is reflectedfrom the sample. In certain embodiments, the rotatable filter assemblyincludes a narrow-band interference filter or plurality of narrow-bandinterference filters. In certain embodiments, the rotatable filterassembly includes an edge interference filter or plurality of edgeinterference filters (such as low-pass or high-pass interferencefilters).

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In another aspect, the invention provides a spectroscopic system fordetecting electromagnetic radiation that has passed through or isreflected from a sample including an electromagnetic radiation source; arotatable filter assembly configured to filter a beam of electromagneticradiation produced by the electromagnetic radiation source; a motorcoupled to the rotatable filter assembly; a position detector includingat least one component rigidly attached to the rotatable filterassembly, the position detector is configured to detect an angularposition of the rotatable filter assembly; and an electromagneticradiation detector configured to detect electromagnetic radiation thathas passed through or is reflected from a sample.

In certain embodiments, the rotatable filter assembly is configured torotate about an axis substantially perpendicular to a path of a beam ofelectromagnetic radiation produced by the electromagnetic radiationsource. In certain embodiments, the rotatable filter assembly isconfigured to rotate about an axis non-perpendicular to a path of a beamof electromagnetic radiation produced by the electromagnetic radiationsource at an angle within a range from about 60 degrees to less than 90degrees (e.g., 89.99 degrees). In certain embodiments, the rotatablefilter assembly includes a narrow-band interference filter.

In certain embodiments, the rotatable filter assembly includes aplurality of filters. In certain embodiments, the rotatable filterassembly includes at least three filters.

In certain embodiments, the surface of the filter(s) is parallel to theaxis of rotation of the rotatable filter assembly. In certainembodiments, the filter(s) is angularly tilted about an axisperpendicular to the axis of rotation of the rotatable filter assemblyand the axis normal to the surface of the filter.

In certain embodiments, the spectroscopic system includes a controllerconfigured to adjust a rotational velocity of the rotatable filterassembly. In certain embodiments, the position detector includes anencoder configured to produce at least a first signal including a seriesof digital pulses at a first frequency, each digital pulse correspondingto an angular position of the rotatable filter assembly. In certainembodiments, the first frequency is a clock frequency. In certainembodiments, the encoder is configured to produce a second signal, andthe spectroscopic system includes an encoder signal processing moduleconfigured to combine the first and second signals into a third signal.In certain embodiments, the third signal includes a series of digitalpulses having at least double the first frequency. In certainembodiments, the encoder includes an edge detector configured to detectan edge of each of at least two signals produced by the encoder and tothereby produce a signal including a series of digital pulses having atleast quadruple the first frequency.

In certain embodiments, the encoder is rigidly attached to the rotatablefilter assembly. In certain embodiments, the system includes aspeed-reduction mechanism configured to control a velocity of therotatable filter assembly. In certain embodiments, the speed-reductionmechanism is configured to control the velocity using a digital feedbackcontrol.

In certain embodiments, the encoder is configured to producesignificantly more digital pulses per rotation of the rotatable filterassembly than are necessary to accurately reproduce an analog signalfrom the electromagnetic radiation detector. In certain embodiments, theencoder is configured to digitize the analog signal at a frequencygreater than a Nyquist criterion corresponding to the analog signal. Incertain embodiments, the encoder is configured to digitize the analogsignal at a frequency greater than 5 times the Nyquist criterion. Incertain embodiments, the encoder is configured to digitize the analogsignal at a frequency at least 8 times the Nyquist criterion. In certainembodiments, the encoder is configured to digitize the analog signal ata frequency at least 10 times the Nyquist criterion. In certainembodiments, the encoder is configured to digitize the analog signalwith at least 1000 pulses per rotation of the rotatable filter assembly.

In certain embodiments, the spectroscopic system includes a variablegain amplifier configured to convert a light signal from theelectromagnetic radiation detector into an electrical signal. In certainembodiments, the variable gain amplifier is in communication with theposition detector and is configured to automatically adjust a gainprofile of a signal received from the electromagnetic radiation detectorbased on a detected angular position of the rotatable filter assembly.In certain embodiments, the amplifier is configured to automaticallyadjust a gain profile of a signal received from the electromagneticradiation detector based on a magnitude of the signal.

In certain embodiments, the spectroscopic system includes a processorconfigured to apply a convolution function to a spectral signal from theelectromagnetic radiation detector, thereby enhancing wavelengthstability and/or repeatability, and/or thereby improving signal-to-noiseratio. In certain embodiments, a width of the convolution function is asgreat as possible without altering or broadening spectral features ofthe spectral signal.

In certain embodiments, the spectroscopic system includes a processorconfigured to apply a baseline correction algorithm to a spectral signalfrom the electromagnetic radiation detector, thereby enhancing long-termmeasurement stability.

In certain embodiments, the spectroscopic system includes a plurality ofelectromagnetic radiation sources, thereby enabling detection ofelectromagnetic radiation over a broader spectrum and/or over multiplespectra. In certain embodiments, the plurality of electromagneticradiation sources includes a UV radiation source and an IR radiationsource. In certain embodiments, the spectroscopic system includes ananalog-to-digital acquisition mechanism in communication with theelectromagnetic radiation detector and the position detector, where theanalog-to-digital acquisition mechanism is configured to digitize,store, and/or process data corresponding to the detected electromagneticradiation. The spectroscopic system may include a computer or mayotherwise share input and output with a computer 2802 (e.g., a computerinternal or external to the spectroscopic system), the computerincluding software for digitizing, receiving, storing, and or processingdata corresponding to the detected electromagnetic radiation and/orsignals created by such detected electromagnetic radiation asillustrated in FIG. 28. The computer may also include a keyboard orother portal for user input, and a screen for display of data to theuser. The computer may include software for process control, dataacquisition, data processing, and/or output representation. Thespectroscopic system may include a wireless system for acquisition ofdata and/or system control. For example, the wireless system may allowwireless data transfer from and/or to a computer, allowing wirelessinput and/or output (and/or system control) by/to a user via a userinterface connected to the computer, such as a keyboard and/or displayscreen. The spectroscopic system may also include a battery systemconfigured to enable stand-alone operation capability.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In another aspect, the invention provides a spectroscopic system fordetecting electromagnetic radiation that has passed through or isreflected from a sample, including an electromagnetic radiation source;a rotatable filter assembly configured to filter a beam ofelectromagnetic radiation produced by the electromagnetic radiationsource; a motor coupled to the rotatable filter assembly, anelectromagnetic radiation detector configured to detect electromagneticradiation that has passed through or is reflected from a sample and tooutput a corresponding analog spectral signal; and a position detectorconfigured to detect an angular position of the rotatable filterassembly, the position detector including an encoder configured toproduce at least a first signal including a series of digital pulses ata first frequency, each digital pulse corresponding to an angularposition of the rotatable filter assembly, wherein the encoder isconfigured to produce significantly more digital pulses per rotation ofthe rotatable filter assembly than are necessary to reproduce the analogspectral signal.

In certain embodiments, the encoder is configured to digitize the analogsignal at a frequency greater than a Nyquist criterion corresponding tothe analog signal. In certain embodiments, the encoder is configured todigitize the analog signal at a frequency greater than 5 times theNyquist criterion. In certain embodiments, the encoder is configured todigitize the analog signal at a frequency at least 8 times the Nyquistcriterion. In certain embodiments, the encoder is configured to digitizethe analog signal at a frequency at least 10 times the Nyquistcriterion. In certain embodiments, the encoder is configured to digitizethe analog signal with at least 1000 pulses per rotation of therotatable filter assembly.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In another aspect, the invention provides a spectroscopic system fordetecting electromagnetic radiation that has passed through or isreflected from a sample, including an electromagnetic radiation sourcehaving a variable intensity; a filter assembly configured to filter abeam of electromagnetic radiation produced by the electromagneticradiation source; a position detector configured to detect a position ofthe filter assembly; a controller configured to adjust the intensity ofthe electromagnetic radiation source; and an electromagnetic radiationdetector configured to detect electromagnetic radiation that has passedthrough or is reflected from a sample.

In certain embodiments, the filter assembly is rotatable and theposition detector is configured to detect an angular position of thefilter assembly.

In certain embodiments, the controller is in communication with theposition detector and is configured to adjust the intensity of theelectromagnetic radiation source based on a detected position of thefilter assembly. In certain embodiments, the filter assembly includes afilter having an active portion and an inactive portion and thecontroller is configured to decrease the intensity of theelectromagnetic radiation source when a beam of electromagneticradiation from the electromagnetic radiation source is incident on aninactive portion of the filter.

In certain embodiments, the controller includes a voltage regulator forcontrolling a voltage supplied to the electromagnetic radiation source.

In certain embodiments, the spectroscopic system includes a plurality ofelectromagnetic radiation sources. In certain embodiments, thespectroscopic system includes a plurality of electromagnetic radiationdetectors.

In certain embodiments, the spectroscopic system includes a variablegain amplifier configured to convert a light signal from theelectromagnetic radiation detector into an electrical signal. In certainembodiments, the amplifier is in communication with the positiondetector and is configured to automatically adjust a gain profile of theelectrical signal based on a detected position of the filter assembly.

In certain embodiments, the filter assembly is configured for rotationabout an axis substantially perpendicular to a path of a beam ofelectromagnetic radiation produced by the electromagnetic radiationsource.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In another aspect, the invention provides a spectroscopic system formonitoring electromagnetic radiation that has passed through or isreflected from a sample including an electromagnetic radiation source; afilter assembly configured to filter a beam of electromagnetic radiationproduced by the electromagnetic radiation source; an electromagneticradiation detector configured to detect electromagnetic radiation thathas passed through or is reflected from a sample; and a processor incommunication with the electromagnetic radiation detector, the processorconfigured to: (i) apply a first calibration spectrum to a firstrecorded spectrum obtained from the electromagnetic radiation detector,thereby determining a measure of one or more compounds in the sample;and (ii) modify the first calibration spectrum to account for a baselinevariation of recorded spectra over time using at least a second,subsequent recorded spectrum obtained from the electromagnetic radiationdetector.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In another aspect, the invention provides a system for monitoring aprocess including an electromagnetic radiation source; a filter assemblyconfigured to filter a beam of electromagnetic radiation produced by theelectromagnetic radiation source; a sampling mechanism configured toalternately direct a sample from a first stream associated with themonitored process into a sampling area and direct a sample from a secondstream associated with the monitored process into the sampling area; anelectromagnetic radiation source configured to direct an electromagneticradiation beam from the electromagnetic radiation source to the samplingarea; an electromagnetic radiation detector configured to detectelectromagnetic radiation that has passed through or is reflected fromthe sampling area; and a processor configured to: (i) obtain a firstspectrum corresponding to the first stream; (ii) store the firstspectrum as a baseline spectrum; and (iii) obtain a second spectrum fromthe second stream using the baseline spectrum, wherein the secondspectrum reflects a compositional difference between the first andsecond streams.

In certain embodiments, the sampling mechanism includes a solenoid valvefor switching between the first and second streams. In certainembodiments, the first stream is an input stream to the monitoredprocess and the second stream is an output stream from the monitoredprocess. In certain embodiments, the first stream is an output streamfrom the monitored process and the second stream is an input stream tothe monitored process.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In another aspect, the invention provides a spectroscopic method fordetecting electromagnetic radiation that has passed through or isreflected from a sample, including: filtering a beam from anelectromagnetic radiation source with a rotating filter assembly;detecting an angular position of the rotating filter assembly with aposition detector having at least one component rigidly coupled to therotating filter assembly; intercepting the beam with a sample; detectingthe beam with an electromagnetic radiation detector; and processing aspectral data signal from the electromagnetic radiation detector toproduce chemical information about the sample.

In certain embodiments, the rotating filter assembly is configured torotate about an axis substantially perpendicular to a path of a beam ofelectromagnetic radiation produced by the electromagnetic radiationsource. In certain embodiments, the rotating filter assembly includes anarrow-band interference filter. In certain embodiments, the rotatingfilter assembly includes at least three filters.

In certain embodiments, the position detector includes an encoderconfigured to produce at least a first signal comprising a series ofdigital pulses at a first frequency, each digital pulse corresponding toan angular position of the rotating filter assembly.

In certain embodiments, the method includes digitizing an analogspectral signal from the electromagnetic radiation detector isperformed. In certain embodiments, digitizing is performed at afrequency significantly greater than necessary to accurately reproducethe analog spectral signal; digitizing is performed at a frequencygreater than a Nyquist criterion corresponding to the analog spectralsignal; and/or digitizing is performed at a frequency greater than atleast ten times the Nyquist criterion.

In certain embodiments, a step of applying a convolution function to aspectral signal from the electromagnetic radiation detector is performedto enhance wavelength stability and/or repeatability, and/or to improvesignal-to-noise ratio.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In yet another aspect, the invention provides a spectroscopic method fordetecting electromagnetic radiation that has passed through or isreflected from a sample to produce chemical information about thesample, the method including: filtering a beam from an electromagneticradiation source with a rotating filter assembly; intercepting the beamwith a sample; detecting the beam with an electromagnetic radiationdetector configured to output an analog spectral signal; detecting anangular position of the rotating filter assembly with a positiondetector, the position detector comprising an encoder configured toproduce at least a first signal comprising a series of digital pulses ata first frequency, each digital pulse corresponding to an angularposition of the rotating filter assembly, wherein the encoder isconfigured to produce significantly more digital pulses per rotation ofthe rotating filter assembly than are necessary to reproduce the analogspectral signal; digitizing the analog spectral signal using the firstfrequency as a clock frequency; and processing the digitized analogspectral signal to produce chemical information about the sample.

In certain embodiments, the first frequency is greater than a Nyquistcriterion corresponding to the analog spectral signal. In certainembodiments, the first frequency corresponds to at least 1000 pulses perrotation of the rotating filter assembly (or, alternatively, at least2000, 1500, 1250, 900, 800, 700, 600, or 500 pulses per rotation).

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In yet another aspect, the invention provides a spectroscopic method fordetecting electromagnetic radiation that has passed through or isreflected from a sample to produce chemical information about thesample, including: filtering a beam from an electromagnetic radiationsource with a filter assembly, the electromagnetic radiation sourcehaving a variable intensity; intercepting the beam with a sample;detecting the beam with an electromagnetic radiation detector; detectinga position of the filter assembly with a position detector; adjustingthe intensity of the electromagnetic radiation source; and processingspectral data from the electromagnetic radiation detector to producechemical information about the sample.

In certain embodiments, adjusting the intensity of the electromagneticradiation source is based on a detected position of the filter assembly.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In yet another aspect, the invention provides a spectroscopic method formonitoring electromagnetic radiation that has passed through or isreflected from a sample, the method including: filtering a beam from anelectromagnetic radiation source with a filter assembly; interceptingthe beam with a sample; detecting the beam with an electromagneticradiation detector; applying a first calibration spectrum to a firstrecorded spectrum obtained from the electromagnetic radiation detector,thereby determining a measure of one or more compounds in the sample;and modifying the first calibration spectrum to account for baselinevariation of the recorded spectra over time using at least a second,subsequent recorded spectrum obtained from the electromagnetic radiationdetector.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In yet another aspect, the invention provides a spectroscopic method formonitoring a process, including: directing a first sample from a firststream associated with the monitored process into a sampling area;directing a second sample from a second stream associated with themonitored process into the sampling area; detecting filtered radiationthat has passed through or is reflected from the sampling area;determining a first spectrum corresponding to the first stream; storingthe first spectrum as a baseline spectrum; and determining a secondspectrum from the second stream using the baseline spectrum, wherein thesecond spectrum reflects a compositional difference between the firstand second streams.

In certain embodiments, the first stream is an input stream to themonitored process and the second stream is an output stream to themonitored process. In certain embodiments, the first stream is an outputstream to the monitored process and the second stream is an input streamto the monitored process.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

In yet another aspect, the invention provides a method for increasingthe robustness and/or stability of the measurement, including: obtaininga first spectrum from an electromagnetic radiation detector; applying aclassical least squares analysis to the first spectrum using a principalcalibration matrix to obtain detection values; determining a residualmagnitude by quantifying how well the first spectrum fit the principalcalibration matrix; comparing the residual magnitude to a predeterminedthreshold to determine if a threshold condition exists and, if athreshold condition exists, creating a secondary reference matrix usingthe first spectrum if a secondary reference matrix does not exist and,if the secondary reference matrix exists, adding the first spectrum tothe secondary reference matrix as a row or a column; adding the rows orcolumns of the secondary reference matrix to the principal referencematrix to update the reference matrix; and reapplying a classical leastsquares analysis to a second spectrum from an electromagnetic radiationdetector.

In certain embodiments, the size of the secondary reference matrix ispredetermined. In certain embodiments, determining a residual magnitudecomprises computing a mean of an absolute function of a classical leastsquares fit of the first spectrum; and/or determining a residualmagnitude comprises computing a maximum value of an absolute function ofa classical least squares fit of the first spectrum.

In certain embodiments, the threshold condition exists when the residualmagnitude exceeds a predetermined threshold value; the thresholdcondition exists when the first spectrum is substantially orthogonal tothe principal calibration matrix; the reference matrix comprisesspectral data from a beam of electromagnetic radiation that has notpassed through a sample; the principal calibration matrix comprisesspectrum values corresponding only to substances to be detected; and/orthe principal calibration matrix comprises spectrum values correspondingto substances to be detected and other substances likely to be foundtogether with the substances to be detected.

The description of elements of the embodiments of other aspects of theinvention can be applied to this aspect of the invention as well.

BRIEF DESCRIPTION OF DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a block diagram of a spectroscopic system for detectingradiation according to an illustrative embodiment of the presentinvention.

FIG. 2 is a perspective view of a rotating filter assembly, according toan illustrative embodiment of the invention.

FIG. 3 is schematic of an encoder signal processing circuit, accordingto an illustrative embodiment of the invention.

FIG. 4 is an illustration of signal-to-noise ratio improvement afterapplication of a convolution function to a spectral signal, according toan illustrative embodiment of the invention.

FIG. 5 is a block diagram of a spectroscopic method according to anillustrative embodiment of the present invention.

FIG. 6 is a flow chart of a spectral processing method using an adaptivealgorithm according to an illustrative embodiment of the presentinvention.

FIG. 7 is a flow chart for a spectral processing method using anadaptive algorithm according to an illustrative embodiment of thepresent invention.

FIG. 8 is an illustration of spectral data from an experiment using aspectroscopic method according to an illustrative embodiment of thepresent invention.

FIG. 9 is an illustration of spectral data from an experiment using aspectroscopic method according to an illustrative embodiment of thepresent invention.

FIG. 10 is an illustration of wavelength scale error caused bywavelength shift.

FIG. 11 is a block diagram of a method for correcting for wavelengthscale error according to an illustrative embodiment of the presentinvention.

FIG. 12 is an illustration of residual spectrum due to wavelength errormismatch and a first order difference spectrum.

FIG. 13 is an illustration of first order absorption spectra of variouscompounds in a mid-IR region.

FIG. 14 is an illustration of multi-region, cross-analysis bandselection.

FIG. 15 is a block diagram of a method for multi-region, cross-analysisband selection according to an illustrative embodiment of the presentinvention.

FIG. 16 is an illustration of potential non-linear spectral error causedby wavelength-dependent spectral magnitude variations.

FIG. 17 is a block diagram of a method of multi-region, cross analysisregression according to an illustrative embodiment of the presentinvention.

FIG. 18 is an illustration of programmatically varying light sourceintensity with dead band regions.

FIG. 19 is a block diagram for a method of modulating the intensity ofan electromagnetic radiation source according to an illustrativeembodiment of the present invention.

FIG. 20 is an illustration of synchronizing two EM radiation sources toobtain higher modulation bandwidth, according to an illustrativeembodiment of the present invention.

FIG. 21 is an illustration of the use of two EM radiation sources andtwo detectors for multi-wavelength region analysis, according to anillustrative embodiment of the present invention.

FIG. 22 is a process flow diagram for a spectroscopic method usingsequential, differential measurement according to an illustrativeembodiment of the present invention.

FIG. 23 is an illustration of a method for quadrupling a clock signalfrom an encoder according to an illustrative embodiment of the presentinvention.

FIG. 24 is an illustration of tilted filter, tilted with respect to therotation axis, according to an illustrative embodiment of the presentinvention.

FIG. 25 is an illustration of a configuration employing stacked filterassemblies, according to an illustrative embodiment of the presentinvention.

FIG. 26 is a comparison of measurement stability between (i) system withrotation axis perpendicular to the beam and (ii) system with rotationaxis non-perpendicular to the beam, according to an illustrativeembodiment of the present invention.

FIG. 27 is a graph illustrating the relationship of peak transmissionwavelength of a bandpass interference filter with the incident angle.

FIG. 28 is a block diagram illustrating the spectroscopic system incommunication with a computer and its elements, according to anillustrative embodiment of the present invention.

DETAILED DESCRIPTION

It is contemplated that methods, systems, and processes described hereinencompass variations and adaptations developed using information fromthe embodiments described herein.

Throughout the description, where systems and compositions are describedas having, including, or comprising specific components, or whereprocesses and methods are described as having, including, or comprisingspecific steps, it is contemplated that, additionally, there are systemsand compositions of the present invention that consist essentially of,or consist of, the recited components, and that there are processes andmethods of the present invention that consist essentially of, or consistof, the recited processing steps.

The mention herein of any publication, for example, in the Backgroundsection, is not an admission that the publication serves as prior artwith respect to any of the claims presented herein. The Backgroundsection is presented for purposes of clarity and is not meant as adescription of prior art with respect to any claim.

Headers are used herein to aid the reader and are not meant to limit theinterpretation of the subject matter described.

FIG. 1 is a block diagram of a spectroscopic system for detectingelectromagnetic (EM) radiation according to an embodiment of the presentinvention. An EM radiation source 100, or multiple EM radiation sources100 covering one or multiple wavelength regions, are configured todirect a beam of EM radiation to a filter assembly 104 (as shown, havingthree filters). The EM radiation source 100 may be made out of a heatedfilament, LED type, or any other suitable type. The beam may becollected and collimated by collimating optics 102, which may be madeout of a series of lenses or mirrors. The collimated beam is interceptedby the filter assembly 104, which is shown as viewed from the top.

In certain embodiments, the filter assembly 104 is configured forrotation. The filter assembly 104 may be positioned relative to thecollimated beam from the light source 100 such that the axis of rotationis substantially perpendicular to the path of the beam. Alternatively,the axis of rotation may be fixed such that it is not perpendicular tothe path of the beam, in order to reduce or eliminate back-reflectedlight and/or stray light and/or to further control the wavelengthcoverage.

In certain embodiments, the filter assembly 104 may have at least threefilters (106, 108, 110). Each of the filters may be a narrow-bandinterference filter configured to pass a certain narrowband of the EMradiation incident on it. These individual filters are generallyavailable commercially off-the-shelf. The filter assembly 104 may beconfigured as indicated in FIG. 1 such that the angle (A) between thecollimated beam and a filter varies. In addition, rotation causes thebeam to be incident on the different filters (106, 108, 110) in theassembly. The numbers of filters that can be employed is between one andfour, depending on the measurement or monitoring application. Inparticular, this depends on the number of compounds that need to bemeasured or monitored. The transmission wavelength of each of thefilters can generally be tuned from its original wavelength toapproximately 95% of the original wavelength assuming a maximum of40-degree change in the incident angle. For example, a filter that has anominal (at 90-degree incident angle) transmission wavelength peak at2000 nm can be tuned to approximately 1900 nm (0.95×2000 nm). If thetarget compounds happen to have spectral features within this region(1900 nm-2000 nm), using only this filter is sufficient for themeasurement. Multiple filters are needed for a wider spectral coverage.Multiple filters may also be used to provide spectral coverage ofdistinct wavelength regions, i.e. regions that are not close to eachother on the wavelength scale. For example, one filter may have anominal transmission at 2000 nm and the other at 8000 nm. Continuing onthe beam path in FIG. 1, the filtered beam then passes through a samplecell 112, which may contain a sample. The sample may be gaseous, liquidor solid. Additional optics 114, such as focusing, collimating and/orcollecting optical elements, may be used to increase the EM radiationthroughput or to better manage or direct the EM radiation whennecessary. In the sample cell 112, the beam is intercepted by thesample, which modifies the spectrum of the original beam. Theinteraction may be in the form of absorption, fluorescence or othertypes of light-matter interactions. The beam may be then focused onto anEM radiation detector 116 using focusing optics 114. The EM radiationdetector 116 may be a semiconductor based detector such as siliconphoto-diode, a pyroelectric photo-detector, or other types of EMradiation detectors. Using an amplifier 120, a spectral signal from theEM radiation detector 116 may be turned into an electrical signal, andmay then be converted into a digital signal by an analog-to-digital(A/D) converter 122.

In certain embodiments, the surface of the filter(s) is parallel to theaxis of the rotation or the rotatable filter assembly. With thisconfiguration, the angular coverage of the rotating filter spectrometerstarts from zero incident angle. For example, if a narrow-band filterhas a nominal (zero angle) peak transmission at 2000 nm, the startingwavelength of the spectral coverage of the spectrometer with the surfaceof filter parallel to the axis of the rotation is theoretically 2000 nm.

In certain embodiment, the surface of one, some, or all of the filters2400 (FIG. 24) is angularly tilted about an axis perpendicular to theaxis of rotation of the rotatable filter assembly and the axis normal tothe surface of the filter. It is found that tilting of the filter(s)eliminates or suppresses the back-reflected light or stray light thatmay cause measurement inaccuracy, non-linearity and/or instability andresults in a significant advantage in measurement stability. FIG. 26shows a comparison between the measurement stability of a system havingthe rotation axis perpendicular to the electromagnetic beam (2600) andthe measurement stability of a system having the rotation axis at 87degrees to the electromagnetic beam (2601). Each plot corresponds to a72-hour of zero stability run, specifically, the system was configuredand calibrated for moisture analysis at around 2.7 μm, and the samplegas was dry nitrogen. Both systems employed a least-squares regressionchemometric method to predict the moisture concentration upon a moisturecalibration spectrum. Plots 2600 and 2601 demonstrate the significantadvantage in measurement stability afforded by tilting the filter(s) inthis way.

It is also found that tilting of the filter(s) improves control of thewavelength coverage of the spectrometer, in light of certain filtercharacteristics and mechanical angular coverage of the rotatable filterassembly. For purposes of illustration, and without wishing to be boundby any particular theory, FIG. 27 shows the theoretical relationshipbetween the incident angle and the peak transmission wavelength of anarrowband interference filter with a nominal (zero-angle) peakwavelength of 2000 nm. Configuration “A” illustrates a system with thesurface of the filter parallel to the axis of rotation. Assuming thatthe effective angular coverage from the rotating filter assembly is 0-30degrees, the resulting spectral coverage is approximately 2000 nm-1900nm. On the other hand, in configuration “B”, in which the surface of thefilter is tilted at 10 degrees with respect to the rotation axis, theresulting incident angle coverage with 0-30 degrees rotation is between10-40 degrees, resulting in a spectral coverage of approximately 1980nm-1830 nm, as illustrated in FIG. 27.

In some embodiments, the amplifier 120 is a fixed gain amplifier.Alternatively, the amplifier 120 may have a variable gain.

A digital spectral signal from the A/D converter 122 may be fed to aprocessor 124 in which a real-time digital signal processing algorithm126 is applied. The final outcome of the process may be quantitativechemical composition data, which may be displayed in a display unit 128.

The use of multiple filters such as shown in FIG. 1 (as shown, filters106, 108, and 110) enables wide discrete spectral coverage. For example,one filter may cover the near infrared region around 2000 nm, and theothers may cover the mid-infrared region around 8000 nm. The EMradiation detector 116 and the EM radiation source 100 may produce asignal at extremely different magnitudes in the different spectralregions. For example, when using an amplifier 120 with a fixed gain, asignal from the 8000 nm may amount to 1 volt, whereas a signal from the2000 nm may amount to 1000 volts due to the higher EM radiation sourceoutput and better detector responsivity at the near infrared region.

In certain embodiments, an amplifier 120 switches the gain based upon anangular position of the filters as commanded by the gain selector logic134. In another embodiment, an amplifier 120 switches the gain basedupon the magnitude of a spectral signal itself.

The A/D convertor 122 receives its timing or clock signal from anencoder 208 that is preferably attached rigidly to the filter assembly104. The encoder 208 may produce digital pulses that correspond to anangular position of the filter assembly 104.

FIG. 2 shows an encoder 208 in more detail. The encoder 208 may includean encoder electronics unit 206 for carrying the digital pulsescorresponding to an angular position of a rotating filter assembly 104.An example of such an off-the-shelf encoder is EM-1-1250 made by USDigital (Vancouver, Wash.), which produces 1250 pulses per quadraturechannel per rotation.

The encoder pulses may be sent to an encoder signal processing unit 130,of which a simplified schematic of one embodiment is shown in FIG. 3. Anencoder signal processing unit 130 may include two Schmitt triggers(304, 306) that reject any glitches or noise due to electromagneticinterference. The clean digital pulses may then be sent to an XOR gate308 to combine the two quadrature signals (300, 302) from an encoderinto a single signal 310 that is doubled in frequency. For example, insome embodiments, an encoder 208 produces 1250 quadrature pulses perrotation. Upon exiting the encoder signal processing unit 130, thesignal 310 has a frequency at least double the frequency of one of thequadrature signals (300, 302) to become at least 2500 pulses perrotation. The encoder signal processing unit 130 enables greaterover-sampling and data averaging that thereby improves wavelengthstability and the system's signal-to-noise ratio.

In certain embodiments, the pulses are multiplied further in frequencyby employing a different electronics scheme. For example, to quadruplean original clock frequency, the following scheme may be used. As shownin FIG. 23, an edge detector 2304 may be used to trigger a short pulseby detecting either a positive or negative edge in the incoming signalsfrom a first and second channel (2300, 2302). The signals from the firstand second channels (2300, 2302) are sent to the edge detector 2304 andthen the same signal is delayed to create the effect of a difference,which will be seen by the edge detector 2304 as a reason to output ahigh logic. The high state may last the duration of the delayintroduced. The duration of the pulse may be long enough to allow an A/Dconvertor 122 to perform a conversion. Illustrated in FIG. 23 is animplementation of this using logic gates. Other methods include the useof multiple combinations of logic gates, flip-flops, logic gates andpassive components, and analog components to achieve similar purpose.The delay can be implemented using multiple gates which are intended toincrease the pulse width out of the edge detector.

With reference to FIG. 3, at least one component of an encoder 208 maybe rigidly attached to a filter assembly 104. This may help to ensurethat there is no mechanical compliance (or “play”) between the twoelements, thus providing for a more stable and repeatable timing orclock signal position regardless of variations in environmentalconditions, such as vibration and temperature variations. Such anarrangement may also enable the use of a speed reduction mechanism, suchas gear or belt drives, to optimize power transmission while maintainingrepeatable and stable clock signal positions.

The filter assembly 104 may include a table 202 for mounting filters (asshown in FIGS. 2, 106, 108, and 110). Each filter may be secured to thetable 202 with a mounting bracket 200. The table 202 may be coupled to ashaft 204.

In certain embodiments, a speed-reduction mechanism may be coupled tothe motor that drives the filter assembly 104. A speed-reductionmechanism may be a belt-and-pulley type, which may provide smooth,noise-free motion. In certain embodiments, the velocity of the rotatingfilter assembly 104 is adjusted and controlled using a digital feedbackcontrol.

To further improve wavelength stability/repeatability, the measuredsignal just before the A/D conversion is over-sampled, i.e. the signalis digitized at a frequency significantly higher than the Nyquistcriterion which is required to accurately reproduce the analog signaldigitally. Such over-sampling is achieved by employing an encoder thatprovides a large number of pulses per rotation. To illustrate byexample, if the to-be-measure spectral features require a clock signalof 100 pulses per rotation, the encoder 208 should be designed or chosensuch that it provides significantly more than 100 pulses per rotation. Asuitable encoder for this example is one that provides on the order of1000 pulses per rotation. The upper limit would be the maximum allowablesampling frequency of the A/D converter 122.

This signal over-sampling may be combined with a digital convolutionstep performed in the processor 124. The combination of dataover-sampling and convolution would improve the wavelength stability orrepeatability and the spectral signal-to-noise ratio. The convolvingfunction 402 may be a “boxcar” function, triangular function, Gaussianfunction or other applicable functions. For the purpose of wavelengthstability improvement, the exact type of convolving function 402 is lessimportant than the width of the function. The width of the convolvingfunction 402 should be maximized to the point where widening it furtherwould alter or broaden the actual spectral features of the measuredcompound. For example, FIG. 4 shows a “box-car” convolution function 402applied to a raw signal 400. It can be seen that the convoluted signal404 is shown with an improved signal-to-noise ratio without loss of anyof the relevant spectral features.

A common source of measurement instability is baseline instabilities ofthe recorded spectrum, which may be due to slight optical alignmentchanges (for example due to temperature variations), light sourcedegradation, dirty optics, etc. FIG. 5 shows a block diagram schematicof a spectral processing method using a baseline correction algorithm toproduce a processed spectrum 502. A baseline correction algorithm 500may be employed to ensure long-term measurement stability. In oneembodiment, a polynomial fit is applied to the spectrum A linear or asecond order fit is generally sufficient to remove common types ofbaseline variations, although a higher order fit may also be used aslong as it does not remove the relevant spectral features. In anotherembodiment, a spectral differentiation is used to remove the baselinevariations. The spectral differentiation algorithm is of the formS_new(n)=S(n+1)−S(n), or variations thereof, where S_new is theresulting baseline-corrected spectrum, S is the original spectrum, and nis the data element of the spectrum.

As shown in one embodiment, shown in FIG. 6, to produce the actualmeasurement values, i.e. the compounds' concentration or density values,a classical least squares analysis 600 is applied to the processedspectrum 502. With this method, a calibration spectra (the “K” matrix)602, described below, is needed before hand. This matrix contains thecalibration spectra of all of the target compounds (compounds to bemeasured).

In continuous monitoring applications in which the instrument cannot bere-zeroed (“zero” or background spectrum taken) frequently, there may bebaseline variations that cannot be completely fitted by a polynomialfunction. This is particularly true with a filter-based spectroscopysystem of the present invention, which tends to be more susceptible tothese type of baseline errors due to the nonlinear wavelength-anglefunction. In addition, spectral variations that are due to un-modeledinterferences such as those due to other unknown compounds may also bepresent, which would also cause measurement instabilities.

In one embodiment, shown in FIG. 6, the present invention overcomesthese problems as described below. An adaptive algorithm is designed;one that continuously and automatically modifies the calibration spectra(“K” matrix) to account for any un-modeled spectral variation includingthose associated with long-term drifts or instabilities. FIG. 6 shows aflow chart of this algorithm showing its basic operation. The processedspectrum (raw spectrum upon passing through convolution and linearbaseline correction) is fitted with a calibration matrix containing theoriginal or the principal calibration matrix (calibration spectra of thetarget compounds) 606 and a secondary calibration matrix 604. Asecondary calibration spectrum is added to the secondary calibrationmatrix each time the magnitude of the residual spectrum from the CLSanalysis exceeds a certain predetermined threshold value, as indicatedby a decision step 618 in the flow chart. The rows or columns of thesecondary calibration matrix 604 are added 608 to the rows or columns ofthe principal calibration matrix 606 to obtain a modified calibrationspectra or “K” matrix 602.

To illustrate by means of an example, consider a principal calibrationmatrix containing three target compounds: k_(A)(λ) for target compoundA, k_(B)(λ) for target compound B, and k_(C)(λ) for target compound C,where k(λ) is essentially a spectrum of the target compound calibratedat a certain compound concentration or density value. When a disturbanceoccurs such that the measured spectrum could not be adequately modeledby the principal calibration matrix (as quantified by the residualmagnitude or spectrum 614, determined by employing the step of residualcomputation 612), the measured spectrum s_(n)(λ) is added to thecalibration matrix. Thus, the calibration matrix 602 becomes:

$\begin{pmatrix}{k_{A}(\lambda)} \\{k_{B}(\lambda)} \\{k_{C}(\lambda)} \\{s_{n}(\lambda)}\end{pmatrix}\begin{matrix}\; & \; \\\; & \; \\\; & {{Secondary}\mspace{14mu} {calibration}} \\{\; } & {spectrum}\end{matrix}$

where n=1, 2, 3, . . . .

There is more than one approach to compute the residual magnitude 616from the residual spectrum. For example, in one embodiment, themagnitude computation of the residual spectrum involves computing themean value of the absolute function of the residual spectrum. Othermagnitude computation method may be used, such as calculating themaximum value of the absolute function of the residual spectrum.

In certain embodiments, the size of the secondary calibration matrix 604(the number of the secondary calibration spectra, “n”) is predetermined.In other embodiments, the size of the secondary calibration matrix maybe continuously updated or limited based upon certain variables such aselapsed time of measurement, orthogonality of the secondary calibrationmatrix to the principal calibration matrix, and the magnitude of theresidual spectrum.

The residual magnitude threshold value used in the comparison step“exceed threshold?” 618 may be determined by experimentation, takinginto account factors including the inherent random spectral noise, thenumber of spectral averaging which affects spectral noise, and therequired stability of the measurement.

FIG. 7 shows another embodiment of the adaptive algorithm, in whichanother condition, “sufficient orthogonality” 700 is added before ameasured spectrum is added to the calibration matrix. In thisembodiment, the measured spectrum is tested whether it is sufficientlyorthogonal to each of the principal calibration spectrum. The testinvolves computing the inner dot product of the normalized vectors, s.k, where s is the normalized measured spectrum (not shown) and k is oneof the normalized principal calibration spectra 606. The result would bebetween zero (completely orthogonal) and one (completely parallel). Thistest is important to ensure no spectrum that is considerably parallel toany of the spectra of the target compounds is entered into thecalibration matrix. If that happens, the measurement results of thetarget compounds would be erroneous. An orthogonality test thresholdvalue should be chosen to minimize this risk. In the present embodiment,that threshold value is chosen to be 0.05.

When the orthogonality criterion is not met, the processor 124 may beconfigured to produce a signal (“Provide Warning” 700) that can be usedto alert the user in various ways, including flashing an LED, generatingsounds, displaying messages, etc. The warning signal(s) tells the userthat there are one or more interference compounds that have spectralfeatures similar to one of the target compounds. The algorithm can alsobe designed such that the warning signal provide specific messages as towhich target compound the interference compound is conflicting with.

The usefulness of the approach is demonstrated in the followingexperiment. A test unit was set up to monitor N-Butane gas as the targetcompound, one of the common hydrocarbons of interest in safetymonitoring application. Isopropanol vapor (IPA) and 1,1-Difluoroethanegas were used as the interferents, both of which are commonly usedcleaning compounds. Note that 1,1-Difluoroethane (R-152a) is commonlyused as the main or sole ingredient of “dust-off” electronic cleaningproducts. The high-resolution absorption spectra (1 cm⁻¹ resolution) ofthe compounds between 3200 nm and 3600 nm are shown in FIG. 8. As seen,the spectra 800 are greatly overlapping. If a traditional chemometricmethod, such as the classical least-squares or principal componentanalysis technique is to be used, the spectra of both interferents mustbe entered into the calibration matrix. Otherwise, greatly erroneousreadings would be produced. With an adaptive algorithm, on the otherhand, the calibration matrix needs to contain only one spectrum, whichis the spectrum of the target gas, N-Butane.

1,1-Difluoroethane, one of the test interference gases was sampled byreleasing it from “dust-off” product near in the inlet of the samplingport. Similarly, IPA vapor was sampled by opening a bottle of rubbingalcohol liquid near the sampling inlet. FIG. 9 demonstrates the abilityof the system to compensate for the interfering compounds, IPA vapor and1,1,-Difluoroethane gas. The top graph 900 shows butane concentrationreadings using an adaptive algorithm. The maximum butane concentrationerror was less than 25 ppm, which also quickly disappeared (within twomeasurement cycles). This happened when a large amount of IPA vapor wasintroduced. Note again that the spectrum of IPA vapor was not includedin the calibration matrix. Without the adaptive algorithm, the maximumbutane concentration error would have been more than 600 ppm due to thesame interference release, as can be seen on the lower graph 902.Similarly, the interference compensation technique worked well for theR-152 interference, in which the system exhibited negligible error.

Wavelength Lock

Another source of measurement instability is wavelength scale variationsdue to optical alignment changes, inherent temperature dependence of theoptical interference filter 3 and/or temperature dependence of thecompound's spectral features themselves. Interference optical filterswill shift to longer wavelength with increasing temperature and shorterwavelength with decreasing temperature. The shift is on the order of0.01-0.2 nm/deg. Celsius. For example, a 10 deg. Celsius shift oftemperature could amount to 2 nm of wavelength scale variation, whichwould degrade measurement stability. FIG. 10 illustrates a wavelengthscale error causing an apparent shift in the absorption spectrum of thesample, potentially causing a significant measurement error usingtraditional least-squares, chemometrics approach.

A “wavelength lock” algorithm is used to compensate for the wavelengtherror by wavelength shifting the measurement spectrum prior to the leastsquares prediction. A block diagram depicted in FIG. 11 shows the flowchart of the methodology. The raw spectrum 502, presumed to containwavelength error, is modeled using a classical least squares algorithm(CLS) 600 (explained below) or other similar approach to obtain thespectrum residual, i.e. the “left-over” part of the spectrum that is notfitted by the model. CLS regression involves the following computation:

c=sK ^(T)(KK ^(T))⁻¹

where c is a vector containing the concentration value(s), s is thesample spectrum, and K is the calibration matrix 1106 containingpre-determined basis spectra. The residual spectrum, r, is obtained byperforming the following computation:

r=s−c ^(T) K ^(T)

The presence of wavelength shift causes a classical least-squaresregression (CLS) residual spectrum 612 to have similar featurecharacteristics with a first order difference spectrum 1100. Toillustrate by means of an example, consider the CH₄ spectra shown inFIG. 10. Using the original spectrum as the calibration spectrum (as theK matrix) and the wavelength shifted spectrum as the sample spectrum, s,the resulting residual spectrum, r, is computed and shown in the topfigure 1200 of FIG. 12. The first order difference spectrum of theoriginal spectrum, on the other hand, is shown in the bottom figure 1202of FIG. 12. As seen, the residual spectrum and the first orderdifference spectrum exhibit great similarities, as expected. Inaddition, the magnitude of the residual spectrum, computed in the stepWavelength shift magnitude computation 1102 is proportional to themagnitude of the wavelength shift error. These features are used tocorrect the wavelength shift error as a step Wavelength correction 1104to obtain a wavelength corrected spectrum 1108. This wavelength errorcorrection algorithm may be applied continuously at a predeterminedinterval, as rapid as once every scan.

Other methods may be used to correct the wavelength shift error. Onemethod includes monitoring the location of the peak of the spectrum. Forexample, the spectral peak at ˜3315 nm in FIG. 10 is monitored toindicate the presence of wavelength shift. Such a method provides lowersensitivity to wavelength shift. In addition, the presence of otherspectral features or peaks (such as those due to background orinterfering compounds) may obscure the result.

Multi-Region, Cross-Analysis Regression

To further enhance measurement sensitivity and selectivity, and tominimize the effects of spectral non-linearities, a multi-band,cross-analysis, least-squares regression is used. The least-squaresregression approach uses a single analysis band or region to measure thetarget compound(s), i.e. using a single calibration matrix, K, over acertain wavelength region. On the other hand, the multi-band, cross-bandregression method uses multiple K calibration matrices for a single ormultiple target compounds. To illustrate by means of an example, weconsider an application where the spectroscopic device is used tomeasure the concentrations of methane (CH₄), ethane (C₂H₆) and propane(C₃H₈) vapors in a certain process stream or in the ambient air. FIG. 13shows the first order absorption spectra of the vapors in the relevantmid-infrared region. The traditional approach uses a single regioncovering the whole relevant wavelength region, for example, between 3200and 3500 nm to perform the analysis using a certain chemometricsalgorithm such as CLS, PLS or PCA. One embodiment makes use of multipleregions to perform the analysis, building multiple calibration matricesand using them simultaneously to perform the compound concentrationscomputation. FIG. 14 shows possible separate band regions, regions 1, 2and 3 for the analysis, each containing a calibration matrix “tuned” forthe analysis of one target compounds.

Furthermore, each calibration matrix (at each region) includes themodels for some of all of the other compounds present in the sample, toaccount for their interferences. Following the previous example, thecalibration matrix for C₃H₈ (using region 1) would contain the models toaccount for the spectral features of CH₄ and C₂H₆ located within region1 to minimize the interference or cross-sensitivity effects. FIG. 15illustrates the general approach. For example, analysis of region 1,1502, corresponding to target compound A and intereferents B, C, and D1500, can yield concentration values of compound A, 1506. Similarly,analysis of region 2, 1504, yields concentration value of compound B,1508.

Furthermore, more than one analysis region may be used to compute thevalue of a single compound, resulting in more than one computedconcentration or density values. These computed values may later bepost-processed to produce a single value or other information. Such amethod is particularly advantageous in a highly complex sample mixtures,high-concentration samples or highly scattering samples, wherenon-linear behaviors are present. To illustrate the method by means ofexample, consider an absorption spectrum shown in FIG. 16, inparticular, notice how an increase in concentration or density valueaffect the magnitude of the absorption spectrum. Instead of uniform orconstant magnitude amplification across the wavelengths, theamplification itself is wavelength dependent. As such, a single-regionanalysis using a linear least-squares regression will result in a largeresidual error and will not provide an accurate concentration or densitycomputation. The multi-region, cross-analysis regression method solvesthe problem by breaking the wavelength region into smaller pieces, eachproducing a computed concentration or density value. The concentrationor density values are then post-processed 1700 to produce a singleconcentration or density value and/or other pertinent informationrelated to the state of the sample or measurement 1702. The generalapproach is illustrated in FIG. 17.

Single-Beam Based Correction

A potential source of spectral baseline instability is instrumentalvariations, such as light source degradation and power variations,optics transmission degradation due to dust and particulates, alignmentchanges, etc. The spectral characteristics of most if not all of theinstrumental variations can be modeled from the spectral characteristicsof the single beam spectrum itself. The single-beam spectrum refers tothe transmission spectrum of an EM radiation source and the opticalsystem without the presence of a sample. The predicted spectral featuresof these potential variations (the instrument-correction spectra) arederived from the single-beam spectrum and entered into the calibrationmatrix. The instrument-correction spectra may be in the form of the puresingle-beam spectrum, the derivative(s) of the single-beam spectrumand/or other derivations of the single-beam spectrum.

Background Calibration Method

Certain embodiments of the invention include development of acalibration matrix, in particular, a “background” calibration set,S_(background), i.e. a set containing the spectra of all of theinterfering background samples except for the target compound,separately from the target compound's calibration set, S_(target). Twoseparate calibration sets are produced, one that of the background, andone that of the target compound.

The background calibration set is developed by intentionally varying theconcentration or density levels of the background interfering compounds.The background calibration set may also include the spectral variationsmodel due to instrumental changes such as light source intensitychanges. Similarly, these instrumental variations shall be simulatedintentionally to build a calibration set that completely and accuratelymodels all of the potential variations. Care should be taken to ensurethat the samples used to develop the background calibration set do notinclude any detectable level of the target compounds. A backgroundcalibration matrix is then developed by reducing the spectral variationsin the background calibration set into a smaller orthogonal set ofvariations using PCA (principal component analysis), PLS (partial leastsquares) or other similar methods. In using PLS, the dependent variablesinput would be a vector of zeroes, due to zero values of the targetcompound in all of the samples.

Independently, the calibration set of the target compound is developedby varying its levels of concentration or density within the relevantrange. Care should be taken so as not to introduce any impurities thatmight have interference effects to the recorded spectra. For example, inthe case of infrared gas absorption measurement, nitrogen or helium maybe used as the balance gas in the calibration set development as neitherexhibit any infrared absorption signals.

The background and target compound calibration sets are then combined toproduce the calibration matrix. To illustrate by means of example,suppose 100 spectra are obtained for the background set and 10 spectraare obtained for the target compound set. The complete calibration setwill contain 110 spectra upon which the calibration matrix is developed.

If there is more than one target compound, the previous steps arerepeated for each additional target compound. Interference between thetarget compounds shall be taken into account by including the spectra ofany other target compounds as part of the background calibration set ofthe subject target compound.

Light Source Power Modulation

To further optimize the system components' dynamic range and/or toprovide better detections of compounds having weak signals withoutincreasing the overall power requirement, the EM radiation source(s) 100may be varied in its intensity by varying the source voltage or currentprogrammatically, so as to provide higher or lower intensity dependingon the spectral range that is being analyzed at each particular instant.FIG. 18 shows an example of an EM radiation intensity variation profile.In this example, the EM radiation source intensity is programmed suchthat each filter analysis region uses a constant source intensity. Inanother embodiment, the light source intensity may be varied within eachor any of the filter analysis regions. As the filter assembly 104rotates, the path of the beam of EM radiation will become incident onboth active and inactive portions of each filter. As the beam isincident on an inactive portion, a dead band 1800 is produced. In anembodiment, the EM radiation source 100 is modulated in such a way sothat the power is zero or close to zero in the dead bands 1800, tominimize the average power dissipation.

In certain embodiments, the modulation command is originated in theprocessor 124, which uses the angular position of the filter assembly104 from the encoder electronics 206 to determine the level of sourceintensity to output. The output (generally in the form of a voltage) ofthe processor 124 enters the EM radiation source power electronics,which is configured to vary the voltage (in the case of voltage mode EMradiation source) or current (in the case of current mode EM radiationsource), and thus varying the resulting intensity of the EM radiationsource 100. The processor contains an algorithm which is used to computethe command signal based on the angular position info from the encoder208. A block diagram illustrating the method is shown in FIG. 19. Inanother embodiment, the EM radiation source intensity is switchedbetween various predetermined levels such as low, med and high, usingtrigger based switch electronics, rather than a processor.

One possible limitation to this EM radiation modulation method is thebandwidth of the EM radiation source(s). Traditional black body sourcessuch as those using tungsten or kanthal filaments have low bandwidths,and thus have generally been used for steady state application. However,some of today's black body sources are designed with filament designsthat are capable of bandwidths up to 50 Hz or more. LED (Light EmittingDiode) and SLED (Super Luminescence Diode) light sources are capable ofhigher modulation bandwidths.

In the case of black body sources, the bandwidth capability can beincreased by employing more than one source, synchronized to provide aseries of modulations or pulses at a higher frequency or duty cycle thanobtainable with one source. FIG. 20 illustrates an implementation ofthis concept where two EM radiation sources 100 are used synchronicallyto double the pulse frequency from what is obtainable by a singlesource. In one embodiment, the beams from the EM radiation sources 100are combined through collecting and collimating optical elements anddirected into the rotating filter assembly 104. In another embodiment,the filaments are packaged in a single collecting optics unit, such as aTO-8 package, with or without an integrated collimating optics.

Multiple Light Sources and/or Detectors Covering Multiple WavelengthRegion

More than one EM radiation source 104 and/or EM radiation detector 116may be used to increase the spectral coverage of the system. Someapplications require analysis in multiple wavelength regions that arenot effectively covered by a single EM radiation source and detector. Incontinuous emission monitoring application, for example, the systemneeds to monitor CO, CO₂ and NOx in streams containing high level ofmoisture. While CO and CO₂ are best analyzed in the mid-infrared region,NOx is best analyzed in the UV region due to its large interference withwater vapor spectrum. There is no single EM radiation source or detectorthat can effectively cover the UV and IR regions simultaneously. Awavelength selective device of this invention may be combined withmultiple EM radiation sources and detectors, providing a novelintegrated system. Another example requiring multiple source arrangementis one where the EM radiation sources are LED or SLED types where eachonly covers a certain narrow band region. In such a case, multiplesources may used with a single detector element.

In one embodiment, the EM radiation beams from the sources 100 arecombined using cold/hot mirrors 2104 with a long/short wavelength passfilter or beamsplitter on the source and detector side. The concept isillustrated in FIG. 21. Multiple of these “beam combiner/splitter”elements may be used for more than two sources and/or two detectors. Inanother embodiment, the EM radiation sources 100 are packaged as anintegrated element, producing a single EM radiation beam output.

Stacked Filter Assemblies

Another configuration or feature of the rotating filter spectroscopicsystem is that of stacked filters or filter assemblies, where the two ormore filters or filter assemblies are stacked along the rotation axis.FIG. 25 illustrates the concept of the mechanical layout. One or moresources and detector may be used. When one source or detector is used,the beam may be split or combined using beam-splitters, beam-combiners,cold filters, hot filters and other equivalent functioning optics. Thepurpose of the stacked filters or filter assemblies may include: (i)increasing the number of wavelength bands that can be covered with asingle motor assembly and/or (ii) covering multiple wavelength bandsthat need to use two different sources and/or detectors such as the UVand the IR region using a single motor assembly.

Differential Measurement of Process or Reaction Monitoring

A differential measurement method may be used for monitoring certainprocesses such as filtration, purification, chemical and biologicalreaction where comparison is made between the input and the outputstreams. The concept is illustrated in FIG. 22. The process input andoutput streams can be selected and sampled sequentially at predeterminedintervals using a solenoid valve 2206, as depicted in FIG. 22. Amonitoring system 2204 is used to obtain spectrum from samples takenfrom the input and output streams. A spectrum obtained from one of thestreams (input stream 2202 or output stream 2208) is stored as the“zero” or baseline spectrum. The spectrum obtained from the other streamis then referenced to the zero spectrum. In an absorption spectroscopymeasurement, the absorption spectrum A(λ) is obtained by applying thefollowing mathematical function:

A(λ)=log₁₀ {T _(input)(λ)/T _(output)(λ)}

where T_(input) and T_(output) are the spectra of the input stream andthe output stream respectively, and they may be interchanged in theabove equation. The above method reduces or even eliminates potentialdrift or measurement instabilities due to instrumental and/orenvironmental changes. Furthermore, the method would reduce the effectsof background interferences.

As illustrated in FIG. 28, the spectroscopic system 2801 may include acomputer or may otherwise share input and output with a computer 2802(e.g., a computer internal or external to the spectroscopic system), thecomputer including software for digitizing, receiving, storing, and orprocessing data corresponding to the detected electromagnetic radiationand/or signals created by such detected electromagnetic radiation. Thecomputer may also include a keyboard or other portal for user input, anda screen for display of data to the user. The computer may includesoftware for process control, data acquisition, data processing, and/oroutput representation. The spectroscopic system may include a wirelesssystem for acquisition of data and/or system control. For example, thewireless system may allow wireless data transfer from and/or to acomputer, allowing wireless input and/or output (and/or system control)by/to a user via a user interface connected to the computer, such as akeyboard and/or display screen.

Equivalents

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims. The relevant teachings ofall the references, patents and patent applications cited herein areincorporated herein by reference in their entirety.

1-3. (canceled)
 4. A spectroscopic system for detecting electromagneticradiation that has passed through or is reflected from a sample, thesystem comprising: an electromagnetic radiation source; a rotatablefilter assembly configured to filter a beam of electromagnetic radiationproduced by the electromagnetic radiation source; a motor coupled to therotatable filter assembly; a position detector comprising at least onecomponent rigidly attached to the rotatable filter assembly, theposition detector configured to detect an angular position of therotatable filter assembly; and an electromagnetic radiation detectorconfigured to detect electromagnetic radiation that has passed throughor is reflected from a sample. 5-91. (canceled)